There is known a spark plug, of the type shown in FIG. 11, for ignition in an internal combustion engine such as vehicle engine. This spark plug 1 includes a metal shell 30 formed in a different-diameter cylindrical shape with a small-diameter front part and a large-diameter rear part, a hollow shaft-shaped (cylindrical) ceramic insulator 21 (also referred to as “cylindrical insulator” or “insulator”) surrounded by and fixed in the metal shell 30 (hollow axial hole), a center electrode 5 protruding from a front end of the insulator 21 (i.e. lower side of FIG. 11) and a ground electrode 51 joined to a front end 31 of the metal shell 30 so as to define a spark discharge gap between a front end of the center electrode 5 and a distal end of the ground electrode 51. A threaded cylindrical portion 33 having an outer circumferential surface formed with a screw thread 34 (also just referred to as “thread”) is provided as a small-diameter cylindrical portion on the front part of the metal shell 30. A polygonal screwing portion 37 is provided on the rear part of the metal shell 30. A flanged portion 36 (also just referred to as “flange”) is provided on the metal shell 30 at a position between the polygonal screwing portion 37 and the thread 34. The spark plug 1 is accordingly mounted to the engine by turning the polygonal screwing portion 37, screwing the thread 34 into a plug hole (threaded hole) of the engine, and seating the flange 36 on a peripheral edge of the plug hole. It is herein noted that, when the terms “front” and “rear” are used to indicate the spark plug 1 or its structural components, such as metal shell 30, and parts (or portions) thereof in the present invention, the term “front” refers to a lower side of FIG. 11; and the term “rear” refers to a side opposite the front side (i.e. upper side of FIG. 11).
FIG. 12 is a schematic view of the metal shell 30 (also referred to as “spark plug metal shell”) before being assembled into the spark plug 1 of FIG. 11. A front end portion of the metal shell 30 located adjacent to the front end 31 and in front of the threaded cylindrical portion 33 is annular-shaped (circular annular in shape) and adapted as an annular front end portion 32 extending over a predetermined length (e.g. about 1 to 3 mm) and having a non-threaded outer circumferential cylindrical surface that is made larger in diameter than a core diameter of the thread 34 (see the enlarged area of FIG. 12). An annular inward protrusion 43 is formed circumferentially on an inner circumferential surface 41 of the threaded cylindrical portion 33. A second inner step 44 is provided on a rear end side of the annular inward protrusion 43 such that an inner surface of the second inner step 44 is tapered down and reduced in diameter toward the front. A front-facing surface is formed on a front end part of the insulator 21 and supported on the second inner step 44 via a packing 60 (see the enlarged area of FIG. 11). A body part 39 of the metal shell 30 located in rear of the threaded cylindrical portion 34 has an inner circumferential surface 48 made larger in diameter than the inner surface 41 of the threaded cylindrical portion 34, with a first inner step 46 provided therebetween. A thin annular cylindrical portion 38 (annular crimp portion) is provided at a rear end of the body part 39 and processed into a crimped portion during the assembling. The flanged portion 36, the screwing polygonal portion 37 and the annular cylindrical portion 38 are arranged in this order from the front, thereby constituting the body part 39. The ground electrode (member) 51 before bending is welded to the front end 31 of the metal shell 30.
The above metal shell 30 is conventionally manufactured by producing a metal shell formed body (metal shell formed product) 30f, which has a different-diameter cylindrical shape and structure similar to the metal shell 30 with an axial hole formed therein for insertion of the insulator 21 as shown in FIG. 13, through cold forging process and then subjecting the metal shell formed body 30f to e.g. cutting and threading to form the thread 34 etc. (see, for example, Japanese Laid-Open Patent Publication No. 2009-095854). As the metal shell formed body 30f is similar in shape and structure to the metal shell 30, parts and portions of the metal shell formed body 30f corresponding to those of the metal shell 30 are principally designated by the same reference numerals in FIG. 13 as those in FIG. 12 in the present invention. However, parts and portions of the metal shell formed body 30f distinguished from the corresponding parts and portions of the metal shell 30 are designated by different names. For example, a cylindrical portion 35 of the metal shell formed body 30f to be processed into the threaded cylindrical portion 34 is referred to as “cylindrical intermediate portion 35”; and a front end portion 32 of the metal shell formed body 30f that is located in front of the cylindrical intermediate portion 35 and that is made smaller in outer diameter than the cylindrical intermediate portion 35 is referred to as “annular front end portion 32”. The axial hole is made through the center axis of the metal shell formed body 30f according to the inner circumferential shape of the metal shell 30. As shown in FIG. 13, an inner circumferential surface of the axial hole includes, in the order from the rear (i.e. upper side) to the front, a large-diameter hole region 48a, a first middle-diameter hole region 41a smaller in diameter than the large-diameter hole region 48a, a small-diameter hole region 43a smaller in diameter than the first middle-diameter hole region 41a and a second middle-diameter hole region 41b larger in diameter than the small-diameter hole region 43a. The configuration of this metal shell formed body will be explained in detail later.
FIG. 14 is a schematic view showing changes in the shape of the formed body during the cold forging process until the completion of the metal shell formed body 30f. In FIG. 14, the metal shell formed body 30f is obtained from a short rod-shaped starting raw material S (see the upper left illustration of FIG. 14) through a series of forming steps. After the completion of the metal shell formed body 30f by the final step of the forging process, the metal shell formed body 30f is subjected to cutting as required and thereby obtained as a metal shell cut body. Then, the ground electrode (member) is joined by welding to the front end of the metal shell formed body. The metal shell 30 (finished product) is completed by performing various processing operations to e.g. form thread on an outer circumferential surface of the cylindrical intermediate portion 35 of the metal shell formed body. Depending on the kind of the metal shell, the thread may be formed on the metal shell formed body before the cutting. The spark plug of FIG. 11 is manufactured by inserting the insulator 21 into the axial hole of the metal shell 30 from the rear end side, with the center electrode 5 protruding from the front end of the insulator 21, to bring the annular front-facing surface of the large-diameter part of the insulator into contact with the second inner step 44 of the annular inward protrusion 43 on the inner circumferential surface 41 of the metal shell 30 via the packing 60, and crimping the rear end portion (annular crimp portion) 38 of the metal shell 30 inwardly and frontwardly. The spark discharge gap is set by bending the ground electrode 51.
As mentioned above, the final formed product (metal shell formed body 30f) obtained by the plurality of cold forging steps has a different-diameter cylindrical appearance close to that of the metal shell 30 as shown in FIG. 13. In FIG. 13, the corresponding parts and portions are principally designated by the same reference numerals as those in FIG. 12 as mentioned above. As in the case of the metal shell 30, a rear cylindrical part of the metal shell formed body 30f is provided as a body part 39 with a radially outwardly protruding flanged portion 36. The cylindrical intermediate portion 35, on which the thread 34 is to be formed by the above-mentioned threading step, is provided on the metal shell formed body 30f in front of the body part 39. The annular front end portion 32, which is smaller in outer diameter than the cylindrical intermediate portion 35, is provided on the metal shell formed body 30f in front of the cylindrical intermediate portion 35 over a predetermined range from the front end 31 toward the rear (see the enlarged area of FIG. 13) as corresponding to that of the metal shell. In the metal shell formed body 30f, the large-diameter hole region 48a and the first middle-diameter hole region 41a are defined by an inner circumferential surface 48 of the body part 39 and an inner circumferential surface 41 of the cylindrical intermediate portion 35, respectively, according to the inner circumferential shape of the metal shell. A first inner step 46 is provided between the large-diameter hole region 48a and the first middle-diameter hole region 41a. Further, the small-diameter hole region 43a is defined by an inner circumferential surface of the annular inward protrusion 43. A second inner step 44 is provided between the first middle-diameter hole region 41a and the small-diameter hole region 43a such that an inner surface of the second inner step 44 is tapered down and reduced in diameter toward the front as a supporting surface to support thereon the insulator 21.
The annular front end portion 32 is conventionally formed on the metal shell formed body in front of the cylindrical intermediate portion 35 in the second step of FIG. 15-2 after the first step (forging step) of FIG. 15-1. It is herein noted that the formed bodies of FIG. 14 (illustrations 1-5) correspond to those formed in the respective process steps of FIG. 15 (illustrations 1-5). In the first step, the starting raw material S is subjected to drawing (cylindrical shaping) and hollow shaping as shown in FIG. 15-1. By the drawing, the outer diameter of the front part of the formed body is made close to the outer diameter of the cylindrical intermediate portion 35 to be processed into the threaded cylindrical portion 33 of the metal shell. The inner diameter of the front part of the formed body is adjusted by the hollow shaping. In the second step, the rear part of the formed body is subjected to diameter expansion and hollow shaping as shown in FIG. 15-2. Simultaneously with the diameter expansion and hollow shaping, the cylindrical drawn front part of the formed part is shaped such that the outer circumference of the front end portion of the cylindrical drawn part (see the area P1 in FIG. 14-2 and in FIG. 15-2) corresponds in shape to the annular front end portion 32 that is smaller in outer diameter than the cylindrical intermediate portion 35. This forming operation is performed by the use of a die (i.e. die 200 as shown in FIG. 15-2) having a working surface shaped corresponding to the annular front end portion 32 that is smaller in outer diameter than the cylindrical intermediate portion 35, i.e., by pressing the front end portion of the cylindrical drawn part of the formed body against the working surface of the die. In the third step (see FIG. 15-3), a punch is pushed into the formed body from its rear end side so as to extrude the rear part of the formed body toward the rear and, at the same time, extend the front part of the formed body and thereby form the cylindrical intermediate portion 35. In the fourth step (see FIG. 15-4), a polygonal portion 37 is formed on the formed body. During the fourth step, an inner bottom wall of the formed body is made thinner (see FIG. 15-4). This thin bottom wall is punched out so as to define the small-diameter hole region (annular inward protrusion) 43a by the inner circumferential surface of the cylindrical intermediate portion 35 in the fifth step (see the illustration 5 of FIG. 15). The metal shell formed body 30f is then completed as shown in FIG. 14-5. In the above process, it is necessary in the third step to use a die (203) having an inner circumferential surface (see the area P3 in FIG. 15-3) shaped corresponding to the annular front end portion 32 in front of the outer circumferential surface of the cylindrical intermediate portion such that the annular front end portion 32 can sustain a forging pressure.
By the way, there is no difference in thread appearance and dimensions among metal shells for spark plugs when the metal shells are of same thread diameter and length. However, the inner circumferential profile of the metal shell, i.e., the inner circumferential profile of the metal shell formed body as a semi-finished product of the metal shell, except the large-diameter hole region 48a, varies from product to product as shown in the left and right section views of FIG. 16. More specifically, the axial lengths and positions of the first middle-diameter hole region 41a, the small-diameter hole region 43a and the second middle-diameter hole region 41b change from product to product. It is because the axial lengths and positions of the respective holes 41a, 43 and 41b change as the axial position of the second inner step 44 varies depending on the performance such as heat resistance required for the spark plug. In a metal shell for use in a high combustion temperature engine such as supercharger-equipped engine, for example, the second inner step 44 is located closer to the front end as shown in the right section view of FIG. 16 as compared to that in an engine with no supercharger even when the thread diameter and length are the same. Even when the thread diameter and length are the same, the axial position of the second inner step 44 is slightly changed according to the thermal value of spark plug and according to the kind of the engines and vehicle. Namely, there are a plurality of kinds of metal shells of the same thread diameter and length. Further, the metal shells of the same thread diameter can have different thread lengths. The axial position of the second inner step 44 can be set to various positions among the metal shells. In consequence, there are a plurality of kinds of metal shells depending on the thread length and the axial position of the second inner step 44 even when the metal shells are of the same thread diameter.
In the above forging process in which the annular front end portion 32 is formed in the second step, it is necessary to slightly vary the lengths of the cylindrical front parts of the second-step formed products for manufacturing of the metal shell formed bodies 30f with different axial positions of second inner steps 44 for metal shells of the same thread diameter and length. The reason for this is that, in the case where the axial positions of the second inner steps 44 differ as shown in the left and right section views of FIG. 16 even though the cylindrical intermediate portions 35 are of the same length in the final metal shell formed bodies 30f, the flow conditions of the workpiece materials need to be set differently in the second step according to such differences in the axial positions of the second inner steps 44. To manufacture the formed bodies of the same thread diameter and length but different second inner step positions, the die (lower die 200 of FIG. 15-2 used in the second step for formation of the annular front end portion 32 has to be changed among those having different working parts (working surfaces) according to the lengths of the cylindrical front end portions of the formed bodies. There is thus a need to use a number of dies (in the second step) corresponding to the axial length of the second inner step 44. In addition, there is also a need in the third step to change the die 203 shape to correspond to the annular front end portion 32 since the annular front end portion 32 formed in the second step has to sustain the forging pressure applied in the third step (see the area P3 of FIG. 15-3).
As mentioned above, the metal shells can have a plurality of kinds of thread lengths depending on the performance required for the spark plugs even though the thread diameters of the metal shells are the same. The axial positions of the second inner steps 44 can vary among the metal shells with such different thread lengths. There is thus a need in the conventional cold forging process, in which the annular front end portion 32 is formed in the above-mentioned forming step (second step), to properly use a number of dies corresponding to the length of the thread 34 and the axial position of the second inner step 44 even in the case of manufacturing the metal shell formed bodies with the same thread diameter. There is also a need to use a number of dies corresponding to the shape of the annular front end portion 32 in the third step. This leads to increases in die production and management costs.
In the case of manufacturing the metal shell formed bodies of different kinds where only the axial position of the second inner step 44 varies from kind to kind, it is necessary to replace the die for formation of the annular front end portion 32 even though the thread diameter and length are the same. Such replacement requires complicated operation with delicate adjustment for proper positioning of the die. This leads to a deterioration in the manufacturing efficiency of the metal shell formed body (metal shell) and becomes a cause of increase in the cost of the spark plug.
The present invention has been made to solve the above problems in the conventional manufacturing method of the metal shell formed body. An advantage of the present invention is a method of manufacturing metal shell formed bodies for metal shells by cold forging in which, as long as the annular front end portions of the metal shell formed bodies are the same in outer diameter and axial length, it is possible to reduce the number or kinds of dies required for formation of the annular front end portions and improve the manufacturing efficiency of the metal shell formed bodies when the metal shells are the same in thread diameter but different in second inner step position or slightly different in thread length.